Impacts of climate anomalies on sap flow rates in a montane beech forest in the Italian eastern Alps

Beech is one of the most ecologically and economically important tree species in Europe and in the context of climate change it is expected to expand towards higher and cooler elevation in the Alpswhile facing stress and potential decline at its lower, warmer limits due to increased drought and heat.

Studying this species ecology is therefore essential to understanding its ability to cope with shifts of climatic regimes towards warmer mean temperatures and more frequent climatic anomalies.

This study evaluated the xylematic sap flux response of a beech stand in the eastern Alps at an elevation of 1250 m in the area of Cembra (Trentino, Italy), in relation to the climate anomalies occurred between 2019 and 2024, determined from a 20+ years archive of standard meteorological observations.

Experimental data were collected using Tree Talker (TT+) devices, which allow continuous monitoring of sap flux density (SFD) at hourly step by application of the thermal dissipation technique; probes were installed on 32 trees of similar size and hierarchy divided into three plots.

Raw flux data from individual trees were routinely subjected to data quality check, including the scrutiny of temperature probes correct functioning, data transmission errors, anomalous differences of temperature between heated and reference temperature probes produced by below canopy solar irradiance gradients

We first characterized the temporal variability of SFD from hourly to seasonal scale as well as the spatial one at individual tree scale and across plots.

The functional resistance of individual trees and of the stand during the identified anomalies in air temperature and VPD were analyzed by quantifying the variation in the daily mean and maximumSFD observed at the peak of the climate anomalies compared to pre-event conditions. Similarly, functional resilience was retrieved considering post-event conditions.

SFD differences among plants was large (up tp a factor 5), and to some extent explained by factors such as tree density and topography (slope, aspect). No significative relationships with tree diameter or height were found.

The seasonal and monthly pattern of sap flux resulted in being driven by two fundamental variables: total solar radiation and VPD, the former triggering the flux while the latter modulating its intensity.

Beech trees appeared to be able to maintain stable SFD values during moderate droughts and heat waves but showed a significant reduction (-45%) under more intense anomalies combining drought and heat waves, as in July 2022. Nevertheless, even after even such cases the monitored trees were able to restore pre-anomaly sap flux rates, exhibiting good resilience.